In recent decades accompanying rapid advances in information-oriented society, there have also been rapid technological advances to provide devices and systems for gathering and communicating information. Of these, display devices have been designed for television screens, commercial signage, personal and laptop computers, personal display devices, and phones of all types, to name the most common information sharing devices.
As the increase in the use of such devices has exploded in frequency and necessity by displacing older technologies, there has been a concern that electromagnetic radiation emission from such devices may cause harm to the human body or neighboring devices or instruments over time. To diminish the potential effects from the electromagnetic radiation emission, display devices are designed with various transparent conductive materials that can be used as electromagnetic wave shielding materials.
In display devices where a continuous conductive film is not practical for providing this protection from electromagnetic radiation emission, it has been found that conductive mesh or patterns can be used for this electromagnetic wave shielding purpose.
Other technologies have been developed to provide new microfabrication methods to provide metallic, two-dimensional, and three-dimensional structures with conductive metals. Patterns have been provided for these purposes using photolithography and imaging through mask materials.
In addition, as the noted display devices have been developed in recent years, attraction has increased greatly for the use of touch screen technology whereby a light touch on a transparent screen surface with a finger or stylus can create signals to cause changes in screen views or cause the reception or sending of information, telecommunications, interaction with the internet, and many other features that are being developed at an ever-increasing pace of innovation. The touch screen technology has been made possible largely by the use of transparent conductive grids on the primary display so that the location of the noted touch on the screen surface can be detected by appropriate electrical circuitry and software.
For a number of years, touch screen displays have been prepared using indium tin oxide (ITO) coatings to create arrays of capacitive patterns or areas used to distinguish multiple point contacts. ITO can be readily patterned using known semiconductor fabrication methods including photolithography and high vacuum processing. However, the use of ITO coatings has a number of disadvantages. Indium is an expensive rare earth metal and is available in limited supply. Moreover, ITO is a ceramic material and is not easily bent or flexed and such coatings require expensive vacuum deposition methods and equipment. In addition, ITO conductivity is relatively low, requiring short line lengths to achieve desired response rates (upon touch). Touch screens used in large displays are broken up into smaller segments in order to reduce the conductive line length to provide acceptable electrical resistance. These smaller segments require additional driving and sensing electronics, further adding to the cost of the devices.
Silver is an ideal conductor having conductivity that is 50 to 100 times greater than that of ITO. Unlike most metal oxides, silver oxide is still reasonably conductive and its use reduces the problem of making reliable electrical connections. Moreover, silver is used in many commercial applications and is available from numerous commercial sources.
In other technologies, transparent polymeric films have been treated with conductive metals such as silver, copper, nickel, and aluminum by such methods as sputtering, ion plating, ion beam assist, wet coating, as well as the vacuum deposition. However, all of these technologies are expensive, tedious, or extremely complicated so that the relevant industries are spending considerable resources to design improved means for forming conductive patterns for various devices especially touch screen displays.
A similar level of transparency and conductivity for patterns can be achieved by producing very fine lines of about 5-6 μm in width of highly conductive material such as copper or silver metal or conductive polymers.
Polymers that can be patternwise switched from hydrophobic nature to hydrophilic nature are known for various uses such as making lithographic printing plates, and such polymers typically comprise carboxylic acid, alcohol, or amine functionality that can be initially attached or “protected” by a chemical protective group that renders it hydrophobic and relatively non-reactive. These protecting groups can be removed with specific chemical triggers such as ultraviolet irradiation, a strong acid, or basic conditions and heat. Catalytic acids can be generated by UV light using a wide variety of photoacid generating compounds such as sulfonium or iodonium salts. Once the protecting groups are detached or removed, the carboxylic acid, alcohol, or amine functionality can be available to provide hydrophilicity and to be available for other purposes.
Polymers containing pendant sulfonate groups rather than carboxylic acid, alcohol, or amine groups could be useful because of the high acidity or low pKa and high stability of the resulting pendant sulfonic acid group, making it highly ionized under a wide range of conditions. However, the “protection” of sulfonate groups is known to be a problem because of the tendency for sulfonate esters to be very reactive, making any “protecting” group unstable to a wide variety of chemical reagents and conditions and thus ineffective. Alternatively, the protected sulfonate-containing polymer could readily degrade under normal environmental conditions such as humidity and heat.
The literature relating to sulfonate “protecting” group technology provides so assistance for this problem. Some limited success has been reported from use of hindered alcohols such as neopentyl, cyclopentyl, methyl tetrahydropyranyl alcohols. Beta-halo alcohols having electron withdrawing groups, especially beta-fluoro alcohols that destabilize the transition state of the deprotection step (thus making the protected sulfonate more stable) have also been reported to function under some conditions. The fluorinated benzyl alcohol, α-(trifluoromethyl)benzyl alcohol has been described as showing some possible use as sulfonate protecting group. See, for example, Pauff and S. C. Miller, Journal of Organic Chemistry, 2013, 78, 711-716; L. Rusha and S. C. Miller, Chem. Commun., 2011, 47, 2038-2040; S. C. Miller, Journal of Organic Chemistry, Vol. 75, No. 13, 2010.
While there are some known reactive polymers that can be used to provide electrically-conductive patterns, there is a need for a way to make reactive polymer patterns that can be used for producing thin electrically-conductive lines using less expensive materials and electroless plating techniques in order to achieve a substantial improvement in cost, reliability, and availability of electrically-conductive patterns for various display devices. Moreover, it would be desirable in some applications to use reactive polymers having “protected” pendant sulfonate groups. The present invention addresses this need as described in considerable detail below.